Pore and fracture pressures are the major factors affecting the success of drilling operations. If pressure is not properly evaluated, it can lead to drilling problems such as lost circulation, blowouts, stuck pipe, hole instability, and excessive costs. Unfortunately, formation pressures can be very difficult to quantify precisely where unusual, or abnormal, pressures exist. Abnormal pressure is a subsurface condition in which the pore pressure of a geologic formation exceeds, or less frequently is below, the expected hydrostatic formation pressure. Abnormally high formation pressures typically occur when impermeable rocks such as shales are compacted rapidly during their deposition, so that their pore fluids cannot always escape and must then support the total overlying rock column. Such pressures may occur as shallow as a few hundred meters below the surface or at depths exceeding thousands of meters and can be present in shale/sand sequences and/or carbonate sections.
The occurrence of overpressured zones has classically been determined well after the overpressured zone has been drilled, sometimes with disastrous results. Abnormal pressure can cause a well to blow out or become uncontrollable during drilling, resulting in overwhelming increases in drilling/completion costs. Therefore, the earliest possible determination of the presence of abnormally overpressured subsurface(s) is a paramount piece of information that is of acute interest to the drilling engineer and the operating company. In particular, knowledge of formation pressure during the drilling enables the well operator to make preparations such as increasing the weight of the drill mud column in order to maintain well control and to prevent a blow out (or to resist fluid loss), as zones of overpressure (or underpressure) are penetrated by the drill bit.
Generally, as an overpressured formation is approached, there are marked differences in degree of compaction and porosity of various formation layers. Also, differences in the minerals composition of interstitial fluids typically occur, and the formation pressure may begin to rise and approach or even exceed the bottom hole pressure, thus decreasing the bottom hole pressure differential. If properties, which are affected by such factors, are closely monitored and plotted with respect to depth, an abnormally pressured zone may be identified when a distinct deviation from an average line trending with depth is observed. To this end, the most commonly used methods for formation pressure prediction in logging-while-drilling (LWD) utilize the same phenomena: changes in shale porosity due to burial and deviation from “normal compaction” trend in abnormally pressurized zones.
The most commonly used measurement techniques include resistivity and acoustic (sonic) logs that may be supplemented with gamma ray and/or SP (spontaneous potential) as shale indicators. In complex cases, a gamma-gamma density and neutron (pulse-neutron) logs are also used. Typically, such logging-while-drilling techniques are used to: (1) identify shale layers, (2) assess their porosity, and (3) account for possible complications, such as lithology changes due to diagenesis, water salinity variations, natural or drilling-induced fractures. The measured parameters, such as electric (resistivity), acoustic (travel time) and nuclear (gamma-gamma/neutron-gamma/pulsed neutron), are plotted versus depth and an established normal compaction trend is provided. Any deviation from this trend should be recognized on porosity logs and may be associated with a subsurface pressure anomaly.
All conventional logs, however, suffer from one key weakness—estimated total porosity is not lithology-independent. As known, lithology-dependent methods give a correct answer in shales if: (1) the lithology is known; (2) there are no fracture components; and (3) the pore space fluid is pure water. However, uncertainties in matrix composition, such as (1) mixed lithology and diagenetic alteration, (2) changes in pore water salinity (salt influence or fresh-water entrapment), (3) presence of hydrocarbons, and (4) natural/induced fractures cause calculation of formation pressure to become very challenging, if not impossible in complex reservoir cases. Most importantly, all known techniques have failed in cases of variations in water salinity (presence of salt dome) and fracturing (tectonic faulting/drilling damage).
The resistivity-based logging techniques are most effected by variations in matrix mineralogy. In normally pressured shales (hydro-pressure regime), there is a decrease of conductivity with depth of burial, due to compaction and associated porosity and water loss. This decreasing of shale porosity forms a “normal compaction line” trend in conductivity or resistivity versus depth plots. Generally, such a normal compaction line can be established for a given area. Depending on various factors this line may shift right or left, but the slope remains essentially the same. Shale-reading points that fall to the left (higher conductivity area) of the normal compaction line are often associated with zones of abnormally high pressure. However, a basic assumption in the use of resistivity logs is that water salinity in shales remains the same, and therefore resistivity changes could be correlated to actual porosity variations. This assumption becomes invalid if there are variations in salt deposition, which may facilitate or impede water conductivity, thus rendering resistivity-based porosity values incorrect.
The above problem is illustrated in FIG. 1, which is a resistivity log of an overpressured formation from Gulf-of-Mexico area. In the FIG. 1, the normal compaction trend is shown as solid line, and resistivity data as black points. The deviation of resistivity at the depth of 2800 m is associated with increased amount of clay-bound water (hence increased conductivity). In this well, two pressure compartment are identified: ˜2500-3200 m and 3400-3800 m. Reasons for this compartmentalization, however, cannot be determined from the resistivity log, since there is no information on the spectral distribution of the total porosity. Reasons for increasing resistivity at 3300 m are unclear also; they may be due to lithology changes, such as presence of dispersed carbonaceous material, diagenetic alterations in shales, or others.
Acoustic logs are also effected by variations in matrix mineralogy. In normally pressured shales sonic logs generally show a decrease in interval transit time with depth of burial. Since interval transit time is a function of porosity, this decrease indicates that the shale porosity decreases with depth. When depth is plotted on the linear y-axis versus shale interval transit time on the logarithmic x-axis, a straight line can be drawn trough the normal pressure points. Points that fall to the left of the normal line represent a zone of abnormally high formation pressure, as shown in FIG. 2. Such acoustic log is less dependent on water conductivity, but strongly influenced by natural and drilling-induced fractures that may drive calculated porosity values too high. Moreover, both acoustic and density methods are strongly influenced by gas presence.
FIG. 2 illustrates acoustic (sonic) log in an overpressured formation from Gulf-of-Mexico area. A normal compaction trend derived from the acoustic log is shown as the smooth line. Deviations to the left are due to increasing of shale porosity and hence are associated with undercompaction in overpressured formations. Top of pressure seal is observed at depth of about 3220 meters, where deviation of actual data points (circles) from the normal trend becomes apparent. Pressure seal is a gradual change in porosity related to pressure increase, also known as transitional zone. Overpressured formation zone marked by constant values in travel time, and started at approximately 4000 meters.
Difficulties associated with the prior art are further related to the fact that often pressure predictions from various conventional logs disagree with each other. FIG. 3, for example, illustrates such disagreement in pore pressure estimates from resistivity and acoustic logs. Logging-while-drilling resistivity measurements (circles) show a long transitional zone in the 9,000-10,000 feet-interval, which is typical for damaged shaly pressure seal. Such a long transitional zone is probably associated with diagenetic alterations and corresponding variations in water salinity. In contrast, acoustic log (small dots in the midsection of the drawing) shows fairly steep changes, meaning that pressure seal is in good condition. Also, fracture gradient (dashed line), calculated based on pore pressure is in disagreement with leak-of test results (EMW—equivalent mud weight) line. Moreover, below 11000 feet calculations based on resistivity seriously overestimates formation pore pressure.
Therefore, it is an object of the present invention to develop a method and system for real-time analysis of shales and accurate porosity-based formation pressure estimation to facilitate early warning of the existence of overpressure. In particular, it is desirable for the new method to enable lithology-independent porosity measurements—that is, at least a portion of the extracted data is related only to shale micro-porosity, wherein diagenetic changes associated with shale composition are not interfering with the desired data. Additionally, it is desirable that at least a portion of the data can provide other information about a reservoir or shale zone. Additionally, it is desirable for the system to have mechanical strength and measurement sensitivity to withstand shock, vibration and erosion associated with drilling and to enable logging and measurement while drilling of the underground formation in most complex underground conditions.